The present invention relates generally to power transfer systems for controlling the distribution of drive torque between the front and rear drivelines of a four-wheel drive vehicle. More particularly, the present invention is directed to a power transmission device adapted for use in motor vehicle driveline applications having a magnetorheological clutch actuator that is operable for controlling actuation of a multi-plate clutch assembly.
In view of increased demand for four-wheel drive vehicles, a plethora of power transfer systems are currently being incorporated into vehicular driveline applications for transferring drive torque to the wheels. In many vehicles, a power transmission device is operably installed between the primary and secondary drivelines. Such power transmission devices are typically equipped with a torque transfer mechanism for selectively and/or automatically transferring drive torque from the primary driveline to the secondary driveline to establish a four-wheel drive mode of operation. For example, the torque transfer mechanism can include a dog-type lock-up clutch that can be selectively engaged for rigidly coupling the secondary driveline to the primary driveline to establish a xe2x80x9cpart-timexe2x80x9d four-wheel drive mode. In contrast, drive torque is only delivered to the primary driveline when the lock-up clutch is released for establishing a two-wheel drive mode.
A modem trend in four-wheel drive motor vehicles is to equip the power transmission device with an adaptive transfer clutch in place of the lock-up clutch. The transfer clutch is operable for automatically directing drive torque to the secondary wheels, without any input or action on the part of the vehicle operator, when traction is lost at the primary wheels for establishing an xe2x80x9con-demandxe2x80x9d four-wheel drive mode. Typically, the transfer clutch includes a multi-plate clutch assembly that is installed between the primary and secondary drivelines and a clutch actuator for generating a clutch engagement force that is applied to the clutch plate assembly. The clutch actuator can be a power-operated device that is actuated in response to the magnitude of an electric control signal sent from an electronic controller unit (ECU). Variable control of the control signal is typically based on changes in current operating characteristics of the vehicle (i.e., vehicle speed, interaxle speed difference, acceleration, steering angle, etc.) as detected by various sensors. Thus, such xe2x80x9con-demandxe2x80x9d power transmission devices can utilize adaptive control schemes for automatically controlling torque distribution during all types of driving and road conditions.
Currently, a large number of on-demand transfer cases are equipped with an electrically-controlled clutch actuator that can regulate the amount of drive torque transferred to the secondary output shaft as a function of the value of the electrical control signal applied thereto. In some applications, the transfer clutch employs an electromagnetic clutch as the power-operated clutch actuator. For example, U.S. Pat. No. 5,407,024 discloses an electromagnet that is incrementally activated to control movement of a ball-ramp drive assembly for applying a clutch engagement force on the multi-plate clutch assembly. Likewise, Japanese Laid-open Patent Application No. 62-18117 discloses a transfer clutch equipped with an electromagnetic actuator for directly controlling actuation of the multi-plate clutch pack assembly.
As an alternative, the transfer clutch can employ an electric motor and a drive assembly as the power-operated clutch actuator. For example, U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having a transfer clutch equipped with an electric motor that controls rotation of a sector plate which, in turn, controls pivotal movement of a lever arm that is operable for applying the clutch engagement force to the multi-plate clutch assembly. Moreover, Japanese Laid-open Patent Application No. 63-66927 discloses a transfer clutch which uses an electric motor to rotate one cam plate of a ball-ramp operator for engaging the multi-plate clutch assembly. Finally, U.S. Pat. Nos. 4,895,236 and 5,423,235 respectively disclose a transfer case equipped with a transfer clutch having an electric motor driving a reduction gearset for controlling movement of a ball screw operator and a ball-ramp operator which, in turn, apply the clutch engagement force to the clutch pack.
While many on-demand clutch control systems similar to those described above are currently used in four-wheel drive vehicles, a need exists to advance the technology and address recognized system limitations. For example, the size, weight and electrical power requirements of the electromagnetic coil or the electric motors needed to provide the described clutch engagement loads may make such system cost prohibitive in some four-wheel drive vehicle applications. In an effort to address these concerns, new technologies are being considered for use in power-operated clutch actuator applications such as, for example, magnetorheological clutch actuators. Examples of such an arrangement are described in U.S. Pat. Nos. 5,915,513 and 6,412,618 wherein a magnetorheological actuator controls operation of a ball-ramp unit to engage the clutch pack. While such an arrangement may appear to advance the art, its complexity clearly illustrates the need to continue development of even further defined advancement.
Thus, its is an object of the present invention to provide a power transmission device for use in a motor vehicle having a torque transfer mechanism equipped with a magnetorheological clutch actuator that is operable for controlling engagement of a friction clutch.
As a related object, the torque transfer mechanism of the present invention is well-suited for use in motor vehicle driveline applications to control the transfer of drive torque between a first rotary member and a second rotary member.
It is a further object of the present invention to provide a magnetorheological clutch actuator having a piston disposed in a piston chamber for engaging a multi-plate clutch assembly, a pump for supplying magnetorheological fluid to the piston chamber, and an electromagnetically-controlled flow control system for use in a torque transfer mechanism.
According to a preferred embodiment, the torque transfer mechanism includes a housing fixed for rotation with the input member and which has a piston chamber formed therein, a piston slidably disposed within the piston chamber and selectively engageable with a clutch pack of the multi-plate clutch assembly, and a pump in fluid communication with the piston chamber and which is operably disposed between the input member and the output member. The occurrence of a rotational speed differential between the input member and the output member causes the pump to generate a pumping action for pumping a magnetorheological fluid through a flow circuit between the pump and the piston chamber. An electromagnet can be selectively energized for varying the viscosity of the magnetorheological fluid flowing in the flow circuit downstream of the piston chamber to induce a back pressure within the piston chamber, thereby inducing axial movement of the piston for engaging the clutch pack.
In operation, activation of the electromagnet creates a magnetic field which passes through the magnetorheological fluid within a flow passage in the flow circuit for changing its viscosity and restricting flow through the flow passage. The restricted flow induces the back pressure within the piston chamber, thereby inducing axial movement of the piston. The piston pushes against a pressure plate to exert a clutch engagement force on the clutch pack. Upon deactivation of the electromagnet, a return spring releases the clutch pack from engagement and acts to axially move the piston back to a neutral position.